Rio to La Paz

Was wondering what the best option is for travelling from Rio to La Paz overland? The cheapest option is preferred. However, I have done long trips on train/bus before and I know it can be horribly draining and boring. So, are there any towns we should stop over in for a night? and if we were to go to the pantanel, how long would you recommend and what are the names of some good organised tours? Or is it easy enough to go and enjoy on your own accord? Any advice much appreciated, plans are pretty flexible.